Check the instruments tab in the browser (F5 if not open already on the right edge) in Studio One first. Search for "Session" and should come up as "SessionDrummer X64" if scanned already.
If not, you just need to make sure that Cakewalk is in your VST scan path. To do this...
Studio One (menu in SO) -> Options... ->Locations->VST Plugins (last tab)
Be sure the Cakewalk VST path is listed (for me is "C:\Program Files\Cakewalk\Vstplugins")
Also be sure the "Scan at startup" is checked at the top (SO does not have a manual scan). Close/reopen SO, and SessionDrummer X64 should show up in your Instruments.